The art of injection molding has been developed primarily for low viscosity materials, i.e., materials having a viscosity of less than about 1 kilopoise. Traditional injection molding equipment and techniques are intended to accomodate thermoplastic materials which can be melted to yield low viscosities. A more recent approach called reaction injection molding is intended for thermosetting materials which have not been highly filled and also have low viscosities. It has been commonly understood that materials having a viscosity greater than about 8 kilopoise, including materials possessing a degree of fill greater than about 40% by weight, were unsatisfactory to process. Simply put, injection molding of high-viscosity materials has been limited by the unavailability of suitable equipment.
Accordingly, a main object of the invention is the provision of an improved apparatus for injection molding highly viscous materials.
Among the deficiencies of prior art equipment as applied to processing high-viscosity materials is the tendency for the flow of material to stagnate and/or clog in the flow channels of the processing apparatus. Another object of the invention is to overcome this deficiency of the prior art.
To make injection molded parts, several unit operations (e.g. mixing, degassing, injecting) are required and it is important to have control over each operation in order to consistently produce acceptable products. Prior art machines that perform two or more operations simultaneously on a given quantity of material do not permit the desired degree of control over all of the operations. It is another object of the invention to produce an apparatus for injection molding high-viscosity materials in which each unit operation is performed separately and therefore permits a high degree of control to be exercised over the parameters that affect the individual operations.
In injection molding of highly filled materials, use of excessive driving forces on the material produces shear stresses in the material that are so high that the filler separates from the binder and thereby produces binder-rich, defective products. This is a particular problem in machines where the injecting forces are applied in a direction normal to the resultant fluid flow. It is a further object of the invention to provide an injection molding apparatus which avoids necessity for application of excessive forces to the material being injected.
High-viscosity materials tend to entrap air during mixing and transport operations, and entrapped air must be minimized before the material is molded in order to ensure that the proper amount of material is dispensed and to avoid undesired air pockets in the molded products. Still a further object of the invention is to provide an injection loading apparatus having improved degassing capabilities.
Prior art equipment also suffered from the disadvantage of being low in production volume capacity. Yet a further object of the invention is the provision of an injection loader having improved productivity. Still other objects and advantages of the invention will appear from the following detailed description which, together with the accompanying drawings, discloses a preferred embodiment of the invention for purposes of illustration only. For definition of the scope of the invention, reference will be made to the appended claims.